Efficiency and opto-electrical studies of solution processed bulk heterojunction organic photovoltaic devices

Kotane, Lesias Morake
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Endeavours to provide safe, clean and renewable energy have sustained considerable interest in photovoltaic (PV) technologies. The potential to harvest and harness solar energy, with an abundance of available materials, has spurred on the research in the fabrication and characterisation of organic photovoltaic (OPV) devices. Given their solution processability, OPV devices bear the promise of a large scale cost effective production process. Furthermore, the potential of producing lightweight and flexible energy sources broadens the range of applications of these devices. However, many challenges still remain in as far as the envisaged widespread usage of OPVs is concerned. Performance related inadequacies based on poor power conversion efficiencies (PCEs) and environmental stability, have thus far prevented the full commercialisation of organic solar cells. This scenario has necessitated the collation of a clearer understanding of the physics underpinning the fabrication and the operation of these devices. In this work, molecular engineering mechanisms of organic photoactive layers where different parameters including the choice of materials and their mass ratios were investigated. The working principles of fabricated polymer:fullerene bulk heterojunction (BHJ) devices were studied to deepen our understanding of the physics of organic solar cells. Three different approaches were selected for this purpose viz. the fabrication and opto-electrical characterisation of (i) P3HT:PC71BM devices, (ii) ternary devices based on the donor:acceptor1:acceptor2 configuration i.e. P3HT:PC61BM:PC71BM and (iii) P3HT:PC61BM devices with nitrogen-doped multi walled carbon nanotubes (N-MWCNTs) added in the photoactive layer. These photoactive blends completed a working OPV device by being sandwiched between a transparent conducting oxide (ITO) –the anode and aluminium (Al) –the cathode. The optimisation of device fabrication processes, was key to the efficient operation of devices. Optical absorption and current-voltage measurements provided the bedrock on which this work was carried out. PCEs, with their dependence on the morphology and charge carrier transport dynamics of devices, were not only extracted from the J–V experimental data but were also compared to establish best performing devices. This was done by establishing current limiting mechanisms in devices where interface potential barriers were compared using the Fowler-Nordheim (FN) and the Richardson-Schottky (RS) emission models and with bulk transport properties estimated from space charge limited current (SCLC) models. Noting that recombination processes are a major contributor to the reduction in generated photocurrents leading to lower PCEs, the physics of charge carrier recombination was studied by focussing on the dependencies of the FF, Voc and Jsc on incident light intensity. The study on P3HT:PC71BM devices focussed on the identification of the optimum mass ratio between the electron donor material (P3HT) and the electron acceptor material (PC71BM). Additionally, kinetic and energetic properties of the optimised blend were probed. The best PCE was measured for the 1:0.8 blend and kinetically and energetically, for the device annealed at 50 οC for 10 minutes. The ternary blend that gave the highest PCE was of the form P3HT:PC61BM:PC71BM ≡ 1:0.4:0.6. The study on the inclusion of N-MWCNTs in to the photoactive layer of P3HT:PC61BMdid not yield the expected increase in the PCE. Instead, it highlighted the importance of a thorough, appropriate and effective preparation of CNTs for their inclusion in OPV devices
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy